Many satellite earth stations are required to operate with no on-site personnel during routine operations. The PAA is particularly well suited to this requirement in that it has no moving parts, and its functions can be automated and/or remotely controlled. Moreover, the PAA can generate multiple simultaneous contact beams for transmitting or receiving. In applications where simultaneous contact with multiple satellites is required (earth terminal gateways for low altitude multiple satellite systems such as Iridium, GlobalStar and Teledesic), the PAA is advantageous because of cost advantages and operational simplicity.
The satellite earth station must, in these example systems, provide a consistent quality of service (gain divided by system noise temperature, G/T, and effective isotropic radiated power, EIRP) over hemispheric coverage range, above a specified critical minimum elevation angle.
For most satellite operating frequency bands, atmospheric attenuation is a significant loss factor that drives the design requirements of the antenna system. For low elevation angular paths, more loss is encountered since the path through the atmosphere itself is longer compared with high elevations (&gt;30.degree. or so). The PAA may be designed to provide more gain at low elevation angles so that the atmospheric losses are approximately compensated.
A phased array antenna utilizing fixed, planar, apertures and designed to provide electronic beam scanning throughout a hemisphere, requires a minimum of three apertures or faces. All array elements constituting the three faces operate in concert to produce multiple simultaneous beams, each capable of nearly hemispheric coverage. As viewed from any aspect, all visible faces participate in beam forming. In this manner, several faces may participate in the generation of any particular transmit or receive beam. Where individual elements and arrays of elements are capable of significant gain at large angular offsets from the normal to the array surface, such elements are useful in contributing to beam gain. From many viewing angles several faces of a multifaceted phased array are visible, and all may combine their constituent elements to form beams.
The prior art includes many teachings regarding various antenna configurations which provide beam steering capabilities. U.S. Pat. Nos. 4,384,290 to Pierrot et al. and 3,699,574 to O'Hara et al. illustrate circular antenna arrays that are positioned on the skin of an airborne vehicle. U.S. Pat. Nos. 4,896,160 and 5,034,751 to Miller, Jr. illustrate the use of planar phased arrays on airborne vehicles. U.S. Pat. Nos. 2,029,015 to Bohm, 2,352,216 to Melvin et al., and 1,640,534 to Conrad all disclose wire antenna systems that enable beam steering actions. U.S. Pat. No. 3,340,530 to Sullivan et al. discloses a directional antenna array which comprises a plurality of corner reflectors having triangular shaped radiators. U.S. Pat. No. 3,648,284 to Dax et al. illustrates various phased array configurations and, in particular, a two radiating phased arrays which enable bi-lateral beam operation.
U.S. Pat. No. 4,922,257 illustrates a phased array configuration wherein the antenna elements are positioned on a hemisphere. Such an antenna shape illustrates the drawbacks of a number of phased array configurations, in that their aperture size varies from a maximum when a considering a source at zenith, to a minimum, when considering a source at the horizon. More specifically, the cross-section of the antenna structure shown in '257 patent exhibits a circular cross-section when approached from zenith but only a semi-circular cross-section when approached from horizon. As a result, the elevation versus gain characteristic of such an antenna is mismatched to low attitude satellite applications.
Japanese published patent application 58/70181 of Toshitsuna illustrates a phased array system wherein, in one configuration, three radiating faces are rotated mechanically while the beams directed from the individual faces are electronically scanned. The Toshitsuna phased array antenna scans in the vertical dimension only and uses mechanical rotation for azimuth tracking.
U.S. Pat. No. 3,564,552 to Fraizer, Jr. discloses a phased-array antenna that is configured in the form of a square-based pyramid. Such a pyramidal antenna shape experiences a substantial variation in aperture cross-section with azimuth. Generally, only two out of four of such an antenna's surfaces are useful when the beam angles are at or near the horizon.
Accordingly, it is an object of this invention to provide an improved phased array antenna configuration that exhibits maximal aperture cross-section at low beam angles.
It is another object of this invention to provide an improved phased-array antenna whose design enables the achievement of a gain characteristic that does not fall below a predetermined threshold, for all beam angles from zenith to a critical minimum elevation angle.